This invention is related to refractive eye surgery and specifically regarding guiding a laser beam through free space using a set of rotatable mirrors into a hand piece module that converts the laser beam into a two dimensional random overlapping scanning parallel laser beam that is delivered to the eye for ablation of cornea tissue to reshape the cornea of the eye.
About three-quarter of the refractive power of the eye is determined by the curvature of the anterior surface of the cornea and changing the shape of the cornea offers a way to significantly reduce or eliminate refractive errors of the eye. The stroma is thick enough that portions of its anterior region can be ablated away to change its profile and thus change the refractive power of the eye for corrective purposes while leaving plenty of remaining stroma tissue.
Various lasers have been used for ophthalmic applications including the treatment of glaucoma, cataract and refractive surgery. For refractive surgery (or corneal reshaping), ultraviolet (UV) lasers, such as excimer lasers at 193 nm and fifth-harmonic Nd:YAG at 208-213 nm have been used for large area surface corneal ablation in a process called photorefractive keratectomy (PRK) and for large area stroma ablation in a process called laser assisted in situ keratomileusis (LASIK). Corneal reshaping may also be performed by laser thermal coagulation currently conducted with Ho:YAG laser using a fiber-coupled contact and flying laser spots non-contact type process.
Refractive surgery has reached an acceptable level of consistency and proficiency due to the development of the excimer laser and fifth harmonic solid state laser used to photo ablate the corneal tissue to reshape the cornea. In my prior U.S. Pat. No. 5,480,396 dated Jan. 2, 1996 for LASER BEAM OPHTHALMOLOGICAL SURGERY METHOD AND APPARATUS and U.S. Pat. No. 5,599,340 dated Feb. 4, 1997 for LASER BEAM OPHTHALMOLOGICAL SURGERY METHOD AND APPARATUS, single or plural beams are formed by splitting one laser beam, and using a random scanning pattern of the beams to scan the laser beams over the cornea. Most of the current commercial refractive laser systems use either one or two laser beams and a random scanning pattern.
However, all the refractive laser systems currently available in the market are similar in that they require the patient's eye to align with a fixed delivery laser beam. In this case, the laser cabinet is bulky and a fixed laser delivery arm is used to transfer the laser beam into a random scanning pattern onto the cornea. The laser delivery arm is around one meter in length and the laser beam is turned down under the microscope in order to align the laser beam with the microscope's visual axis. The distance from the lower part of the laser delivery arm to the surface of the cornea is around 250 millimeters. The patient lies down on a table that can be XYZ fine-tuned in order to move the patient's eye to the focusing point of the microscope and laser delivery set up. Using this arrangement, the patient is required to move in order to align the patient's eye to the laser beam. To prevent eye movement during the surgery, the surgeon uses a ring or other tools to stabilize the eye ball. Most of the modern laser systems employ an eye tracker system to follow eye movement but even the most advanced three dimensional eye tracking system cannot guide the laser beam to normal incidence upon the corneal surface due to the rotational nature of the eye ball.
U.S. Pat. No. 5,599,340 describes an UV laser with an XY scanned device that delivers the laser beam from a fixed point to the cornea without any physical contact with eye. The visual axis of a microscope is aligned with the UV laser beam since the microscope is used to monitor the ablation. A sophisticated movement patient table is used to move the patient's eye into alignment with the visual axis of the microscope and the UV laser beam. This requires significant effort for the surgeon to precisely align the beam with the eye.
U.S. Patent Application Publication No. US 2013/0131653 A1 (EMI femtosecond laser application) consists of a femtosecond laser apparatus where the laser beam pulses are sent through an XY-scanning device located in a main cabinet. The beam then travels through a mirror-lens relay optical arm to a hand piece that contains a XYZ piezo stage and a very short focal length lens with a high numerical aperture. The hand piece is connected to the surface of the eye via a suction ring. This apparatus therefore aligns the laser beam with the eye to simplify the surgical procedure by removing the process of aligning the patient using a sophisticated movement patient table to align the eye with the laser beam.
However, the function of a femtosecond laser apparatus in refractive surgery is to cut and create a flap in the cornea whereas the function of a UV laser or fifth-harmonic Nd:YAG system in refractive surgery is to ablate and reshape the cornea. Due to the difference in the nature of the procedure between the two type of systems, a millimeter sized laser spot and a 10 mm ablation area is required for UV (excimer or fifth-harmolic solid state) laser refractive surgery whereas the femtosecond apparatus utilize a significantly smaller 2-5 micron laser spot size and an ablation area less than 10 mm. A key difference to the femtosecond laser is an UV laser's sensitivity to laser loss as its laser path increases. This attribute makes it preferable in current UV laser apparatus designs to deliver the laser source via a fixed arm to shorten the laser path. To compensate for eye movement, all current market leaders utilize an eye tracker to account for the fixed arm delivery's sensitivity to eye movement. A flexible articulated mirror arm lengthens and complicates the UV laser's path and is not an obvious solution to current refractive surgery problems.
The present invention proposes a design for a laser system in which the laser beam aligns with the patient's eye rather than having the patient's eye aligned to a fixed UV laser beam delivery point. The laser system uses a rotatable mirror set module to transfer the laser beam from the laser source into a hand piece module at its normal entrance plane without the use of any lenses along the path. In order to overcome the long UV laser path loss and alignment sensitivity, the present invention uses only mirrors in an optical arm as opposed to mirrors and lenses (mirror-lens relay optical arm). Placing lenses in an optical arm amplifies alignment errors and creates a more complex module. Therefore, the present invention uses a mirror set module containing only mirrors to transmit the light source to simplify the optical system's design and operation. Also, a purged nitrogen gas design is incorporated in the laser beam path to reduce the UV laser traveling loss through air. The hand piece module, which contains a two dimensional scanner (such as a piezo tip/tilt scanner or a set of galvanometer scanners), a f-theta telecentric scan focusing lens and other surgery aiding designs/devices converts the laser beam to a two dimensional parallel scanning laser beam as it exits the hand piece and onto the patient's eye. The f-theta telecentric scan focusing lens both focuses the laser beam and converts it from divergent scanning to parallel scanning. The parallel scanning characteristic improves ablation efficiency along the peripheral of the cornea. In the new system, even if the eye is moving during the surgery, the laser beam remains at normal incidence on the eye due to it's flexible nature. Eye trackers in current systems cannot accomplish this and are unnecessary in our invention.